Conventionally, traveling wheels are generally used for the traveling parts of tractors, but in some cases, traveling crawlers are used (for example, JP-A 2001-253362). In the conventional art, in case of that steering mechanisms such as a reduction gear and a conical link mechanism are connected to the steering wheel, the reduction gear mechanism and the conical link mechanism are mounted as separate units to the steering wheel column. Thereby, a large number of components were required, and the assembling operation was not easy but difficult.